70 female subjects who had attempted to lose weight responded to Rotter's I-E Scale and to a linear scale indicating their success in weight reduction. They also indicated whether they had ever joined an organized weight-reduction program and, if so, whether they had completed it. On the basis of their responses they were classified as Externals or Internals and as Joiners or Non-joiners producing four groups in a 2 × 2 factorial design. The groups were External Joiners, External Non-joiners, Internal Joiners and Internal Nonjoiners. The perceived locus of control did not affect self-rating scores of weight reduction. Nor did membership status affect ratings of success. An interaction between perceived locus of control and membership status was found. Internal Joiners reported greater success in weight loss than Internal Non-joiners. External subjects, however, reported greater success in weight reduction when they did not join a weight-reduction program. The least successful group in weight loss was composed of Joiners who dropped out of a program before its completion.